1. Field of the Invention
This invention relates to new and useful improvements in pneumatically operated impact drilling tools and more particularly to pneumatic drills which enter the bore hole and are known as downhole drills. In particular, this invention relates to improved pneumatically operated drills which maybe operated at a variety of selected flow rates and pressures of the compressed air or other pneumatic fluid used in operation of the tool.
2. Brief Description of the Prior Art
The most pertinent prior art relating to this invention are the prior patents of applicant and of other inventors assigned to the assignee of this invention.
Rosco U.S. Pat. No. 3,896,886 discloses an air hammer embodying an outer housing structure connectable to rotatable drill pipe string through which compressed air is conducted. A hammer piston reciprocates in the housing structure, compressed air being directed alternately to the upper and lower ends of the piston to effect its reciprocation in the structure, each downward stroke inflicting an impact blow upon the anvil portion of an anvil bit extending upwardly within the lower portion of the housing structure. The flow of air to the upper and lower ends of the hammer piston is controlled by valve passages formed in the piston and a relatively stationary air supply tube which closes the passage to the lower end of the piston when the outer housing structure is lifted by the drill pipe string to allow the bit to hang down from the housing during the circulation of air for flushing cuttings from the bore hole.
Curington U.S. Pat. No. 3,944,003 discloses an air hammer embodying an outer housing structure connectable to a rotatable drill pipe string through which compressed air is conducted. A hammer piston reciprocates in the housing structure, compressed air being directed alternately to the upper and lower ends of the piston to effect its reciprocation in the structure, each downward stroke inflicting an impact blow upon the anvil portion of an anvil bit extending upwardly within the lower portion of the housing structure. The compressed air acts against the piston over the full internal cross-sectional area of the housing structure in delivering its impact blow, such compressed air acting downwardly over at least a portion of the piston area during its entire downward stroke, including its latter portion, during which the power or impacting air is being exhausted from the housing structure.
Curington U.S. Pat. No. 3,958,645 discloses an air hammer embodying an outer housing structure connectable to a rotatable drill pipe string through which compressed air is conducted. A hammer piston reciprocates in the housing structure along flexible inlet and outlet tubes, compressed air being directed alternately to the upper and lower ends of the piston to effect its reciprocation in the structure, each downward stroke inflicting an impact blow upon the anvil portion of an anvil bit extending upwardly within the lower portion of the housing structure. The piston contacts the housing structure at the upper and lower portions of the piston only, so that the piston can deviate upon flexing of the housing structure under load, and not bind in the housing structure, as permitted by the flexible inlet and outlet tubes. Excess compressed air is permitted to bypass through an orifice in the piston itself, to assist in cleaning the bore hole of cuttings and to avoid excess back pressure in the tool above the piston, resulting from the excess air delivered by the air compressor.
The patents described above define and claim features of a commercially available pneumatic downhole hammer, viz. the Model D-2 air hammer, manufactured and sold by the assignee of this invention, Baker Drill, Inc. The Baker Drill Model d-2 air hammer is designed to operate at a low air flow for low air pressures and at a high air flow for high air pressures. It has recently been observed that many drillers desire to use compressed air rigs which operate at high pressure but relatively low volume. As a result, there has been a demand for air hammers which will operate with a low volume of compressed air when run at high pressure.